Disulfide cross-links reveal conserved features of DNA topoisomerase I architecture and a role for the N terminus in clamp closure.

In eukaryotes, DNA topoisomerase I (Top1) catalyzes the relaxation of supercoiled DNA by a conserved mechanism of transient DNA strand breakage, rotation, and religation. The unusual architecture of the monomeric human enzyme comprises a conserved protein clamp, which is tightly wrapped about duplex DNA, and an extended coiled-coil linker domain that appropriately positions the C-terminal active site tyrosine domain against the Top1 core to form the catalytic pocket. A structurally undefined N-terminal domain, dispensable for enzyme activity, mediates protein-protein interactions.

Human DNA topoisomerase IB: structure and functions.

Human DNA Topoisomerase I is a 765aa monomeric enzyme composed of four domains: the N-terminal domain, highly charged and responsible for several protein-protein interactions, the core domain that embraces the DNA during catalysis, the highly charged linker domain and the C-teminal domain containing the active site. The enzyme promotes the relaxation of supercoiled DNA by nicking and rejoining one of the strands of the DNA. Its activity is critical for many biological processes including DNA replication, transcription, and recombination.

Alterations in linker flexibility suppress DNA topoisomerase I mutant-induced cell lethality.

Eukaryotic DNA topoisomerase I (Top1p) catalyzes changes in DNA topology via the formation of a covalent enzyme-DNA intermediate, which is reversibly stabilized by the anticancer agent camptothecin (CPT). Crystallographic studies of the 70-kDa C terminus of human Top1p bound to duplex DNA describe a monomeric protein clamp circumscribing the DNA helix. The structures, which lack the N-terminal domain, comprise the conserved clamp, an extended linker domain, and the conserved C-terminal active site Tyr domain.

New aspects of the reactivity of tyrosinase.

Tyrosinase was found to be active in the sulfoxidation of thioanisol, producing the (R)-sulfoxide with high enantiomeric excess. The activity of the enzyme with phenolic and diphenolic substrates in a mixed aqueous Hepes buffer pH 6.8-methanol-glycerol solvent was also investigated over a range of temperatures.

Locking the DNA topoisomerase I protein clamp inhibits DNA rotation and induces cell lethality.

Eukaryotic DNA topoisomerase I (Top1) is a monomeric protein clamp that functions in DNA replication, transcription, and recombination. Opposable "lip" domains form a salt bridge to complete Top1 protein clamping of duplex DNA. Changes in DNA topology are catalyzed by the formation of a transient phosphotyrosyl linkage between the active-site Tyr-723 and a single DNA strand.